Downhole tool control

ABSTRACT

A method of operating a downhole tool in a no-flow configuration in which a piston and a flow-restriction cooperate to occlude a tool throughbore. The tool is reconfigured to an intermediate configuration by flowing fluid through the tool at an intermediate flow-rate lower than an operating flow-rate and axially translating the piston to an intermediate position in which the piston and flow-restriction cooperate to define an intermediate flow area. Axial translation of the piston between a no-flow position and the intermediate position includes an occlusional stage of a first axial extent during which the piston and the flow-restriction occlude the tool throughbore, and a transitional stage of a second axial extent during which the piston and the flow-restriction cooperate to provide a step-change in flow area. The tool is reconfigured to an operating configuration by flowing fluid through the tool at the operating flow-rate and axially translating the piston.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of GB Patent Application No. 1302981.4, filed on Feb. 20, 2013, the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to the control of a downhole tool or device, utilizing fluid pressure or flow.

BACKGROUND OF THE INVENTION

In the oil and gas industry it is well known to utilize variations in flow though a drill string to actuate or control actuation of downhole tools or devices. Flow through a restriction in a spring-loaded sleeve may be utilized to create a pressure differential across the sleeve, the pressure differential moving the sleeve downwards from a first or no-flow position, against the action of the spring, to a second or flow position associated with activation or actuation of an associated device. The movement of the sleeve may be controlled by means of a cam arrangement, such as a J-slot or cam track and cam follower pin. The cam track may be configured to provide for two or more flow positions. One flow configuration may be associated with actuation of an associated device, and in another flow configuration the device may remain inactive. The operator may achieve these positions simply by cycling the surface pumps on and off. However, if the pumps are cycled for other reasons, such as to make a connection at surface, the operator may have to cycle the pumps a number of times to achieve or regain the desired configuration. There is also a risk that the operator will have a mistaken belief that a tool is in a certain configuration when it is not, which may have significant operational and safety implications.

In other arrangements the cam track may define alternative paths. U.S. Pat. No. 6,289,999 describes a fluid flow control device in which an operator may select a path associated with a flow-through mode or a path associated with a valve control mode. The selection is made by the operator remotely tracking the movement of a lug along a ratchet path following the actuation of fluid pumps. At a certain relative position of the lug on the ratchet path, shutting off the fluid pumps causes the lug to track back along the ratchet path, a corner in the ratchet path then deflecting the lug from a first operational mode path into a second path. The relative position of the lug on the ratchet path where shutting off the pumps will result in the change of mode path is described as the “window of opportunity” and is signaled to the operator on surface by fluid pulse signals created by an external flange on an inner piston aligning with flanges on an outer housing as the piston translates relative to the housing. Such an argument requires the operator to actively monitor a pressure gauge and recognize the appropriate fluid pulse signals.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method of operating a downhole tool, the method comprising:

providing a downhole tool in a no-flow configuration in which an axially movable tubular piston and a flow-restriction cooperate to substantially occlude a tool throughbore;

reconfiguring the tool to an intermediate configuration by flowing fluid through the tool at an intermediate flow-rate lower than an operating flow-rate and axially translating the piston to an intermediate position in which the piston and flow-restriction cooperate to define an intermediate flow area, wherein axial translation of the piston between a no-flow position and the intermediate position comprises an occlusional stage of a first axial extent during which the piston and the flow-restriction substantially occlude the tool throughbore and a transitional stage of a second axial extent during which the piston and the flow-restriction cooperate to provide a step-change in flow area;

holding the tool in the intermediate configuration by flowing fluid through the tool at the intermediate flow-rate; and

reconfiguring the tool to an operating configuration by flowing fluid through the tool at the operating flow-rate and axially translating the piston to an operating position in which the tool defines an operating flow area.

The tool may be reconfigured between the different configurations in any appropriate order or sequence.

For example, the tool may be provided in the no-flow configuration and then reconfigured to the intermediate configuration by activating surface pumps to circulate fluid through the tool at the intermediate flow-rate and axially translate the piston from the no-flow position to the intermediate position. In this sequence, the axial translation of the piston will comprise an initial occlusional stage and a secondary transitional stage. The flow-rate may subsequently be reduced to zero to return the tool to the no-flow configuration, or alternatively the flow-rate may be increased to the operating flow-rate to reconfigure the tool to the operating configuration.

Alternatively, or in addition, the tool may be provided in the operating configuration and then reconfigured to the intermediate configuration by reducing the flow-rate through the tool from the operating flow-rate to the intermediate flow-rate and permitting the piston to assume the intermediate position.

Alternatively, or in addition, the tool may be directly reconfigured from the no-flow configuration to the operating configuration by increasing the flow-rate directly from zero to the operating flow-rate without seeking to maintain the flow-rate at an intermediate level and thus attain and then maintain the intermediate configuration. Similarly, decreasing the flow-rate directly from the operating flow-rate to zero may directly reconfigure the tool from the operating configuration to the no-flow configuration.

Thus, embodiments of the invention permit an operator to reliably attain and hold an intermediate configuration of the downhole tool, increasing the activation options available when compared to a conventional flow-activated tool, which is likely to be only reliably maintained in a no-flow configuration and an operating flow configuration. This degree of control may be achieved merely by selecting an appropriate flow-rate of fluid through the tool, typically by control of surface pumps used to circulate fluid through a downhole tubing string incorporating the tool.

The provision of the occlusional stage of axial translation of the piston between the no-flow configuration and the intermediate position may provide the operator with assurance that the piston has moved to the intermediate position and achieved the desired function, for example a tool activation or setting associated with the intermediate tool configuration. In certain embodiments, even a relatively small flow-rate will ensure that the intermediate configuration is achieved. Further, in certain embodiments an intermediate flow-rate within a relatively broad range will achieve the desired intermediate configuration. The interaction of the flow-restriction and the piston serves to facilitate attaining the intermediate configuration from the operating configuration, the step-change in flow area at the transitional stage tending to maintain the tool in the intermediate configuration over a range of flow-rates below the operating flow-rate.

The second axial extent of the transitional stage may be less than the first axial extent of the occlusional stage.

The reconfiguration of the tool to the operating configuration may involve a degree of translation of the piston from the no-flow configuration greater than the degree of translation of the piston from the no-flow position to the intermediate position.

The method may comprise removing the flow-restriction from the tool, moving the restriction out of cooperating engagement with the piston, or otherwise reconfiguring the restriction, to improve access to the tool throughbore below the restriction location or to increase the flow area through the tool. For example, a flow-restricting member provided in the body may be retrievable or otherwise removable.

The order or sequence of reconfiguration of the tool may be controlled or guided, for example a cam or J-slot arrangement may be provided between the piston and a tool body. In one embodiment one element of the tool may define a cam track and another element may define a cam follower, such as a pin. The cam track may define alternative or multiple paths or branches and the path followed may be operator-determined by selecting a particular sequence of configurations.

In one embodiment the method may be utilized to allow selection of a first operating configuration and a second operating configuration, the method comprising:

flowing fluid through the tool at the operating flow-rate and with the tool in a first operating configuration;

reducing the flow through the tool to the intermediate flow-rate and reconfiguring the tool to the intermediate configuration; and

increasing the flow through the tool from the intermediate flow-rate to the operating flow-rate and reconfiguring the device from the intermediate configuration to a second operating configuration.

In an alternative operating sequence, if the flow-rate is reduced directly from the operating flow-rate to zero the tool may be reconfigured directly from the first operating configuration to the no-flow configuration, and if the flow rate is then increased directly from zero to the operating flow-rate the tool may be reconfigured to the first operating configuration. Thus, for example, in normal operations in which the flow-rate is varied directly between zero and the operating flow-rate as the surface pumps are switched on and off, the tool may be cycled between the no-flow configuration and the first operating configuration. Only if the operator selects to operate the surface pumps to provide a particular sequence of flow-rates, for example reducing the flow-rate to the intermediate flow-rate and then increasing the flow-rate to the operating flow-rate, will the tool assume the second operating configuration.

The second operating configuration may be associated with a branch or path of a cam track distinct from a branch or track associated with the first operating configuration.

The method may further comprise:

-   -   ceasing fluid flow through the tool and reconfiguring the tool         from the second operating configuration to the no-flow         configuration, or     -   ceasing fluid flow through the tool and reconfiguring the tool         from the first operating configuration to the no-flow         configuration.

According to another aspect of the present invention there is provided a downhole tool having utility in an operator-selectable intermediate configuration and in an operating configuration, the tool comprising:

a tubular body;

a tubular piston axially movably mounted in the body and defining a through bore, the piston being movable between a no-flow position, an intermediate position and an operating position; and

a flow-restriction cooperating with the piston to vary the flow area of the through bore;

in the no-flow position the piston and flow-restriction cooperating to substantially occlude the through bore;

in the intermediate position the piston and flow-restriction cooperating to define an intermediate flow area, axial translation of the piston between the no-flow position and the intermediate position comprising an occlusional stage of a first axial extent during which the piston and the flow-restriction cooperate to substantially occlude the through bore and a transitional stage of a second axial extent during which the piston and the flow-restriction cooperate to provide a step-change in flow area; and

with the piston in the operating position the tool defining an operating flow area.

The flow restriction may take any appropriate form. In one embodiment the flow restriction may include an elongate flow-restricting member mounted in the body. The flow-restricting member may be coaxial with the piston. The flow-restricting member may be received within the piston. The flow-restricting member may be axially movable relative to the piston.

In one embodiment the flow-restricting member may include a substantially cylindrical portion which cooperates with a complementary passage or restriction in the piston when the tool is in the no-flow configuration and during the occlusional stage of translation between the no-flow position and the intermediate position. The transitional stage of translation occurs when the cylindrical portion of the flow-restricting member and the complementary passage or restriction separate to provide a step-change in flow area. Of course other configurations of flow-restricting member and piston may be utilized to achieve a similar effect, such as a flow-restricting member with a stepped profile which provides the step change in area.

Alternatively, or in addition, the flow restriction may include an elongate flow restriction or probe mounted on the piston which cooperates with a complementary passage or restriction in the body.

The piston may define a flow restriction, for example a nozzle, such that increasing flow through the piston creates an increasing axial fluid pressure force on the piston. If the piston is biased towards the no-flow position, increasing flow may tend to increase the distance of the piston from the no-flow position. In some embodiments the piston flow restriction may cooperate with a body-mounted flow-restricting member. In other embodiments the piston and body may cooperate to define a differential piston, wherein an area of the piston is exposed to internal tool pressure, which may be drill string pressure, and an oppositely directed area of the piston is exposed to external tool pressure, which may be annulus pressure. Accordingly, a higher internal pressure may be utilized to urge the piston towards the operating position.

According to an aspect of the present invention there is provided a method of reconfiguring a downhole device between a no-flow configuration, a first flow configuration and a second flow configuration, the method comprising:

providing a downhole device in a no-flow configuration;

flowing fluid through the downhole device at an operating flow-rate and reconfiguring the device to a first flow configuration;

maintaining fluid flow through the downhole device at the operating flow-rate and maintaining the device in the first flow configuration;

reducing the fluid flow through the downhole device from the operating flow-rate to an intermediate flow-rate lower than the operating flow-rate and reconfiguring the device to an intermediate flow configuration between the no-flow configuration and the first flow configuration; and

increasing the fluid flow through the downhole device from the intermediate flow-rate to the operating flow-rate and reconfiguring the device from the intermediate configuration to a second flow configuration.

The method may further comprise:

-   -   stopping fluid flow through the downhole device and         reconfiguring the device from the second flow configuration to         the no-flow configuration, or     -   stopping fluid flow through the downhole device and         reconfiguring the device from the first flow configuration to         the no-flow configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIGS. 1 a, 1 b, 1 c and 1 d are schematic illustrations of a tool in accordance with a first embodiment of the present invention;

FIGS. 2 a, 2 b and 2 c are schematic illustrations of a tool in accordance with a second embodiment of the present invention;

FIG. 3 is a schematic illustration of a cam track of a tool in accordance with an embodiment of the present invention;

FIGS. 4 a, 4 b and 4 c are schematic illustrations of a tool in accordance with another embodiment of the present invention; and

FIG. 5 is a schematic illustration of a tool in accordance with a further embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is first made to FIGS. 1 a, 1 b, 1 c, and 1 d of the drawings, which are schematic illustrations of a tool 10 in accordance with a first embodiment of the present invention. The tool 10 is intended to form part of a downhole tubular string, such as a drill string, tool string, or the like. Accordingly, the tool 10 includes a cylindrical body 12 having appropriate end connections (not shown) for incorporation in the associated string. Mounted within the body 12 is a tubular piston 14 including an internal flow restriction in the form of a nozzle 16, such that circulation or flow of fluid in the normal direction, that is from surface towards the distal end of the string, creates a fluid pressure force across the piston 14, tending to translate the piston downwards, against the action of a compression spring 18.

A cylindrical flow restriction in the form of a probe 20 is mounted to the body 12 and, with the piston 14 in the raised position (FIG. 1 c), the probe 20 extends into the piston 14 and through the nozzle 16. The probe 20 is coaxial with the piston 14 and has an outer diameter only slightly smaller than the internal diameter of the nozzle 16. Accordingly, while the probe 20 is located within the nozzle 16 the tool body is substantially occluded.

FIG. 1 a illustrates the relative positions of the tool elements in a typical operating configuration, with fluid being pumped through the tool 10 and the associated string at a normal operational rate such as required during, for example, a drilling operation. In this situation the fluid pressure differential created across the nozzle 16 is sufficient to move the piston 14 to its lowermost position in the body 12. It will be observed that the upper end of the piston 14 is spaced from and clear of the probe 20.

FIG. 1 b illustrates the relative positions of the tool elements when the flow-rate of the fluid flowing through the string has been reduced to a relatively low intermediate rate such that there is a minimal if any fluid pressure force created across the nozzle 16 and the upwards spring force on the piston 14 is greater than the opposing downwards forces; the piston 14 thus moves upwards under the influence of the spring 18. However, as the upper end of the piston 14 approaches the lower end of the probe 20, the piston 14 and probe 20 cooperate to create a rapid or step-change restriction in the fluid flow area, the pressure drop resulting from the flow restriction creating a fluid pressure force which maintains the piston 14 in the illustrated intermediate position.

The piston 14 will remain in this intermediate position until the flow-rate is increased to generate a sufficient fluid pressure force across the nozzle 16 to overcome the action of the spring 18 and move the piston 14 downwards towards the operating position as illustrated in FIG. 1 a, or until the flow-rate is reduced to a negligible level and the piston 14 translates upwards towards the no-flow configuration as illustrated in FIG. 1 c.

The configuration of the tool 10 is such that the operator may hold or retain the piston 14 in the intermediate position over a range of flow-rates, and it is not necessary for the operator to achieve a precise flow-rate to cause the piston 14 to hover in the intermediate position, as would be the case in the absence of the interaction between the piston 14 and the probe 20; the step-change in flow area created as the piston 14 and probe 20 move from the intermediate configuration towards the no-flow configuration tends to retain the intermediate configuration over a wider range of flow-rates.

As noted above, with negligible or no flow, the piston 14 moves upwards such that the probe 20 extends into and through the piston nozzle 16.

To return the tool from the no-flow configuration to the intermediate configuration, the operator initiates flow through the string, typically by activating the surface pumps. The intermediate configuration is likely to be achieved simply by turning the pumps up sufficiently to circulate fluid through the string; the occlusion of the tool 10 by the interaction of the piston 14 and the probe 20 causes the piston 14 to move beyond the end of the probe 20 and thus permit a degree of fluid flow. At very low flow-rates the tool 10 may experience a degree of chatter, as the piston 14 moves between a just-open and just-closed position, however this may be avoided by a small increase in flow-rate.

Alternatively, if the surface pumps are simply restored to the normal operating flow-rate the piston 14 will move directly to the operating position, passing through but not remaining in the intermediate position. Similarly, simply shutting the pumps down directly from the normal operating level will cause the tool to move directly from the operating configuration to the no-flow configuration, passing through but not remaining in the intermediate position.

Reference is now made to FIGS. 2 a, 2 b and 2 c of the drawings, these Figures being schematic illustrations of a tool 110 in accordance with a second embodiment of the present invention. The tool 110 operates in a generally similar manner to the tool 10 described above, but includes a number of different features, as will be described.

The tool 110 includes a cylindrical body 112 and mounted within the body 112 is a tubular piston 114 having a cylindrical inner flow surface 115. A stepped-profile cylindrical flow probe 120 is mounted to the body 112 and, with the tool 110 in the no-flow configuration and the piston 114 in the raised position (FIG. 2 c), extends into the piston 114. The probe 120 is coaxial with the piston 114 and has an upper portion 120 a with an outer diameter only very slightly smaller than the internal diameter of the inner flow surface 115 and a lower portion 120 b at the probe free end with an outer diameter significantly smaller than the internal diameter of the surface 115. Accordingly, while the probe upper portion 120 a is located within the piston 114 the tool body is substantially occluded, and while the probe lower portion 120 b is located within the piston 114 fluid may flow through the tool 110.

FIG. 2 a illustrates the relative positions of the tool elements in a typical operating configuration, with fluid being pumped through the tool 110 and the associated string at a normal operational rate such as required during, for example, a drilling operation. In this situation the fluid pressure differential created across the piston 114 and the restriction 130 a resulting from the interaction between the piston inner flow surface 115 and the probe lower portion 120 b is sufficient to move the piston 114 to its lowermost position in the body 112, and fully compress the spring 118. It will be observed that the upper end of the piston 114 is spaced from and clear of the probe upper portion 120 a.

FIG. 2 b illustrates the relative positions of the tool elements when the flow-rate of the fluid flowing through the string has been reduced to a relatively low level such that there is a minimal fluid pressure force created across the restriction 130 a. The upwards spring force on the piston 114 is thus greater than the opposing downwards forces and the piston 114 moves upwards under the influence of the spring 118. However, as the upper end of the piston 114 approaches the step or transition 120 c between the upper and lower probe portions 120 a, 120 b, the piston 114 and probe transition 120 c cooperate to create a rapid or step-change restriction in the fluid flow area, the pressure drop resulting from the resulting restriction 130 b creating a fluid pressure force which maintains the piston 114 in the illustrated intermediate position.

The piston 114 will remain in this intermediate position until the flow-rate is increased to generate a sufficient fluid pressure force across the restriction 130 b to overcome the action of the spring 118 and move the piston 114 downwards, or until the flow-rate is reduced to a negligible level and the piston 114 translates upwards towards the no-flow configuration as illustrated in FIG. 2 c.

As with the tool 10 described above, the configuration of the tool 110 is such that the operator may retain the piston 114 in the intermediate position over a range of flow-rates, and it is not necessary for the operator to achieve a precise flow-rate to cause the piston 114 to hover in the intermediate position, as would be the case in the absence of the interaction between the piston 114 and the probe transition 120 c; the step-change in flow area created as the piston 114 and probe 120 move from the intermediate configuration tends to retain the configuration over a wider range of flow-rates.

With negligible or no flow, the piston 114 moves upwards such that the entire probe 120 extends into the piston 114 and the upper probe portion 120 a substantially occludes the piston 114.

To return the tool 110 to the intermediate configuration as illustrated in FIG. 2 b, the operator initiates flow through the string, typically by activating the surface pumps. The intermediate configuration is likely to be achieved simply by turning the pumps up sufficiently to circulate fluid through the string; the occlusion of the tool 110 by the interaction of the piston 114 and the probe upper portion 120 a causes the piston 114 to move beyond the end of the probe transition 120 c in the presence of a relatively low flow rate.

The upper end of the probe 120 includes a wireline overshot profile 132 and a probe-mounting spider 134 which secures the probe 120 to the body 112 via shear pins 136, thus permitting removal of the probe 120 from the tool 110 if desired. Retrieval of the probe 120 removes the bore restriction created by the probe 120 and also provides unrestricted access to the string bore below the tool 110.

Reference is now made to FIG. 3 of the drawings, which is a schematic illustration of a cam track 50 of a tool, such as one of the tools 10, 110 as described above, in accordance with an embodiment of the present invention. As will be described, this embodiment permits an operator to configure the tool in two distinct operating configurations.

The cam track 50 is formed on the inner diameter of the tool body 12, and a cam follower pin 52 extends radially outwards from the piston 14. In normal flow conditions the piston 14 is urged downwards with the interaction of the pin 52 and track 50 holding the piston 14 in a first position 1 corresponding to a first operating configuration, for example as illustrated in FIG. 1 a. If the fluid flow is then reduced to the intermediate flow-rate the pin 52 moves up the track 50 to a second position 2, corresponding to the intermediate configuration, as illustrated in FIG. 1 b of the drawings. If the flow is held at the intermediate flow-rate for a short period, given the manner in which the piston 14 and probe 20 interact, the operator can be confident that the intermediate configuration has been achieved. If the pumps are then brought up to normal flow, the pin 52 will travel back down the track 50 but will move into a blind track branch 50 a and to a third position 3, which permits the piston 14 to be translated further downwards to a second operating configuration, as illustrated in FIG. 1 d of the drawings. This extra stroke may be utilized to perform a desired tool activation, for example to actuate or extend a cutting blade on a reaming tool.

However, if the tool is in the first operating configuration, with the pin 52 in position 1, and flow is stopped completely, the pin will move directly from position 1 to a fourth position 4, corresponding to the no-flow configuration, as illustrated in FIG. 1 c. If the pumps are then restarted the pin 52 will move to the next position 1, corresponding to the first operating configuration. Accordingly, if the pumps are stopped, for example to make a connection at surface, and then restarted, the tool 10 will not be activated. If activation is required the flow-rate must be reduced to and preferably held at the intermediate flow-rate and then increased again without stopping the pumps.

Reference is now made to FIGS. 4 a, 4 b and 4 c of the drawings, schematic illustrations of a tool 210 in accordance with another embodiment of the present invention. The tool 210 operates in a manner which is generally similar to the tools 10, 110 described above but has some different constructional features, as will be described.

Mounted within the tool body 212 is a tubular piston 214 including an internal nozzle 216, such that flow of fluid through the tool in the normal direction, that is from surface towards the distal end of the drill string incorporating the tool 210, creates a fluid pressure force across the piston 214, over seal area at seal diameter B, tending to translate the piston downwards, against the action of a compression spring 218. A cylindrical probe 220 is mounted on the upper end of the piston and cooperates with a restriction 215 provided in the body 212 above the piston 214. With the piston 214 in the raised position (FIG. 4 c), the probe 220 extends into the restriction 215. The probe 220 has an outer diameter only slightly smaller than the internal diameter of the restriction 215. Accordingly, while the probe 220 is located within the restriction 215 the tool body is substantially occluded.

FIG. 4 a illustrates the relative positions of the tool elements in a typical operating or drilling configuration, with fluid being pumped through the tool 210 at a normal operational rate, for example while drilling. In this situation the fluid pressure differential created across the nozzle 216 is sufficient to move the piston 214 to its lowermost position in the body 212 in which the lower end of the body restriction 215 is spaced from and clear of the upper end of the piston probe 220. The relatively large seal area at least diameter B minimized the pressure drop required across the nozzle 216, thus minimizing the pump pressure required to maintain the piston in the operating or drilling position.

FIG. 4 b illustrates the relative positions of the tool elements when the flow-rate of the fluid flowing through the string has been reduced to a relatively low, intermediate rate such that there is minimal if any fluid pressure force created across the nozzle 216 and the upwards spring force on the piston 214 is greater than the opposing downwards forces; the piston 214 thus moves upwards under the influence of the spring 218. As the upper end of the probe 220 approaches the lower end of the restriction 215, the probe 220 and the restriction 215 cooperate to create a rapid or step-change restriction in the fluid flow area. The fluid pressure force now acting over diameter A, corresponding to the diameter of the probe 220, maintains the piston 214 in the illustrated intermediate position.

The piston 214 will remain in this intermediate position until the flow-rate is increased to generate a sufficient fluid pressure force across the nozzle 216 to overcome the action of the spring 218 and move the piston 214 downwards towards the operating position as illustrated in FIG. 4 a, or until the flow-rate is reduced to a negligible level and the piston 214 translates upwards to the no-flow configuration as illustrated in FIG. 4 c.

Reference is now made to FIG. 5 of the drawings, which illustrates a tool 310 in accordance with a further embodiment of the invention. The tool 310 is illustrated in the intermediate or reduced flow position, with fluid pressure acting over area at seal diameter X [[A]], the area of the upper end of the piston probe 320. It will be noted that the tool 310 is similar to the tool 210 described above in a number of respects. However, the piston 314 does not feature an internal nozzle. Rather, movement of the piston 314 to the operating position is achieved utilizing differential pressure, as described below.

The piston 314 carries external seals [[B]] 321, [[C]] 322 which engage the inner wall of the body 312, the volume between the seals [[B]] 321, [[C]] 322 being in communication with the tool exterior. A port to annulus 324 is provided in body 312. Accordingly, in use, the volume will be in communication with the annulus. The upper seals [[B]] 320 describe a larger diameter than the lower seals [[C]] 322.

When flow through the string is increased to the normal operating rate the pressure differential between the interior of the tool and the drill string, at pressure P1, and the annulus surrounding the drill string, at pressure P2, will increase due to drill bit pressure losses and the like. This pressure differential will act on both seal areas at seal diameters Y [[B]] and Z [[C]], and because seal area at seal diameter & [[B]] is larger than seal area at seal diameter Z [[C]] the piston 314 will move down within the body 312 to the operating position. However, there is no pressure drop in the fluid flowing through the piston 314 such that there is no increase in pump pressure required at the operating fluid flow rate.

It will be apparent to those of skill in the art that the above-described embodiments are merely exemplary of the present invention.

It will also be apparent that the advantages provided by the various embodiments of the present invention are applicable to many different tools and devices. For example, the ability to reliably achieve and maintain a fluid flow or pressure activated tool in an intermediate position or configuration provides additional functionality to tools which previously offered only two configurations (a no-flow configuration and a flow configuration). 

What is claimed is:
 1. A method of operating a downhole tool, the method comprising: providing a downhole tool in a no-flow configuration in which an axially movable tubular piston and a flow-restriction cooperate to substantially occlude a tool throughbore; reconfiguring the tool to an intermediate configuration by flowing fluid through the tool at an intermediate flow-rate lower than an operating flow-rate and axially translating the piston to an intermediate position in which the piston and the flow-restriction cooperate to define an intermediate flow area, wherein axial translation of the piston between a no-flow position and the intermediate position comprises an occlusional stage of a first axial extent during which the piston and the flow-restriction substantially occlude the tool throughbore and a transitional stage of a second axial extent during which the piston and the flow-restriction cooperate to provide a step-change in flow area; holding the tool in the intermediate configuration by flowing fluid through the tool at the intermediate flow-rate; and reconfiguring the tool to an operating configuration by flowing fluid through the tool at an operating flow-rate and axially translating the piston to an operating position in which the tool defines an operating flow area.
 2. The method of claim 1, wherein the second axial extent is less than the first axial extent.
 3. The method of claim 1, comprising providing the tool in the no-flow configuration and then reconfiguring the tool to the intermediate configuration by pumping fluid through the tool at the intermediate flow-rate and axially translating the piston from the no-flow position to the intermediate position, the axial translation of the piston comprising an initial occlusional stage and a secondary transitional stage.
 4. The method of claim 3, comprising subsequently reducing the flow-rate to return the tool to the no-flow configuration.
 5. The method of claim 3, comprising subsequently increasing the flow-rate to the operating flow-rate to reconfigure the tool to the operating configuration.
 6. The method of claim 1, comprising providing the tool in the operating configuration and then reconfiguring the tool to the intermediate configuration by reducing the flow-rate through the tool from the operating flow-rate to the intermediate flow-rate and permitting the piston to assume the intermediate position.
 7. The method of claim 1, comprising reconfiguring the tool from the no-flow configuration to the operating configuration by increasing the flow-rate directly from zero to the operating flow-rate.
 8. The method of claim 1, comprising reconfiguring the tool from the operating configuration to the no-flow configuration by decreasing the flow-rate directly from the operating flow-rate to zero.
 9. The method of claim 1, wherein reconfiguring the tool to the operational configuration involves a degree of translation of the piston from the no-flow position greater than the degree of translation of the piston from the no-flow position to the intermediate position.
 10. The method of claim 1, comprising removing the flow-restriction from the tool.
 11. The method of claim 1, comprising controlling the sequence of reconfiguration of the tool by a cam arrangement.
 12. The method of claim 1, comprising: flowing fluid through the tool at the operating flow-rate and with the tool in a first operating configuration; reducing the flow through the tool to the intermediate flow-rate and reconfiguring the tool to the intermediate configuration; and increasing the flow through the tool from the intermediate flow-rate to the operating flow-rate and reconfiguring the tool from the intermediate configuration to a second operating configuration.
 13. The method of claim 12, comprising reducing the flow-rate directly from the operating flow-rate to zero and reconfiguring the tool directly from the first operating configuration to the no-flow configuration, and then increasing the flow-rate directly from zero to the operating flow-rate to reconfigure the tool to the first operating configuration.
 14. The method of claim 12, further comprising: ceasing fluid flow through the tool and reconfiguring the tool from the second operating configuration to the no-flow configuration.
 15. The method of claim 12, further comprising: ceasing fluid flow through the tool and reconfiguring the tool from the first operating configuration to the no-flow configuration.
 16. A downhole tool having utility in an operator-selectable intermediate configuration and in an operating configuration, the tool comprising: a tubular body; a piston axially movably mounted in the body, the piston being movable between a no-flow position, an intermediate position and an operating position; and a flow restriction cooperating with the piston to vary a flow area of the tool; wherein, in the no-flow position, the piston and the flow-restriction cooperate to substantially occlude the through bore; wherein, in the intermediate position, the piston and the flow-restriction cooperate to define an intermediate flow area, axial translation of the piston between the no-flow position and the intermediate position comprising an occlusional stage of a first axial extent during which the piston and the flow-restriction cooperate to substantially occlude the through bore and a transitional stage of a second axial extent during which the piston and the flow-restricting member cooperate to provide a step-change in flow area; and wherein, with the piston in the operating position, the tool defines an operating flow restriction.
 17. The tool of claim 16, wherein the piston is biased towards the no-flow position.
 18. The tool of claim 16, wherein the flow restriction includes an elongate flow-restricting member mounted in the body.
 19. The tool of claim 18, wherein the flow-restricting member is coaxial with the piston.
 20. The tool of claim 18, wherein the flow-restricting member is received within the piston.
 21. The tool of claim 18, wherein the flow-restricting member includes a substantially cylindrical portion which is received in a complementary passage in the piston when the piston is in the no-flow position and during the occlusional stage of translation between the no-flow position and the intermediate position.
 22. The tool of claim 16, wherein the flow restriction includes an elongate flow restriction mounted on the piston which cooperates with a complementary restriction in the body.
 23. The tool of claim 16, wherein the piston defines a piston flow restriction, such that increasing flow through the piston creates an increasing axial fluid pressure force on the piston.
 24. The tool of claim 23, wherein the piston flow restriction cooperates with a body-mounted flow-restricting member.
 25. The tool of claim 16, wherein the piston and body cooperate to define a differential piston, wherein an area of the piston is exposed to internal tool pressure and an oppositely directed area of the piston is exposed to external tool pressure.
 26. The tool of claim 16, comprising a cam arrangement for controlling the movement of the piston relative to the body.
 27. A method of reconfiguring a downhole device between a no-flow configuration, a first flow configuration and a second flow configuration, the method comprising: providing a device in a no-flow configuration; flowing fluid through the device at an operating flow-rate and reconfiguring the device to a first flow configuration; maintaining fluid flow through the device at the operating flow-rate and maintaining the device in the first flow configuration; reducing the fluid flow through the device from the operating flow-rate to an intermediate flow-rate lower than the operating flow-rate and reconfiguring the device to an intermediate flow configuration between the no-flow configuration and the first flow configuration; and increasing the fluid flow through the device from the intermediate flow-rate to the operating flow-rate and reconfiguring the device from the intermediate configuration to a second flow configuration.
 28. The method of claim 27, further comprising: stopping fluid flow through the device and reconfiguring the device from the second flow configuration to the no-flow configuration, or stopping fluid flow through the device and reconfiguring the device from the first flow configuration to the no-flow configuration.
 29. The tool of claim 16, wherein the piston is a tubular piston defining a throughbore and the flow area varied by the flow restriction cooperating with the piston is a flow area of the throughbore of the tool.
 30. The tool of claim 22, wherein the elongate flow restriction mounted on the piston is received in a complementary restriction in the body when the piston is in the no-flow position. 